The field of the present invention is rocker arm assemblies for internal combustion engines and more particularly, rocker arm assemblies for internal combustion engines that can selectively switch between high lift and low lift valve operation.
Rocker arms transmit motion from a rotating cam shaft to a stem of a poppet valve to open and close the valve. Almost universally, the valve is spring-biased shut and the cam via the rocker arm controls the opening and closing of the valve. One type of rocker arm is the finger follower rocker arm.
In recent times, finger follower and other types of rocker arms have been made to selectively totally or partially deactivate to allow enhanced control of vehicle engines in regard to emissions and fuel economy.
In one such rocker arm assembly, the rocker arm has an outer body that engages the valve stem and an inner, lost motion arm pivotally mounted on and within the outer body for movement relative to the outer body. The lost motion arm is spring-biased upward against an overhead engine cam center lobe to be pivoted by the same. A latch mechanism with an extendable plunger is positioned within the outer body. The plunger is normally in a position to limit movement of the lost motion arm relative to the outer body so that the cam lobe can pivot the outer body and lost motion arm together. as an integral unit to activate the valve stem. Withdrawal of the latch mechanism plunger allows the lost motion arm to freewheel in a lost motion manner without causing partial or full movement of the outer body and valve stem.
In Diggs et al. U.S. Pat. No. 5,960,755, commonly assigned and incorporated herein by reference, a rocker arm assembly and method of utilization for high lift and low lift exhaust valve operation is disclosed. (The terms long duration and short duration are sometimes substituted for the terms high lift and low lift.) In the rocker arm assembly of Diggs et al., the duration that the exhaust poppet valve is open and the valve""s maximum displacement during its opening can be selectively changed. For the high lift operation, a cam lobe makes contact with the lost motion arm while the lost motion arm is prevented from rotating relative with the outer body by the latch plunger. When shorter duration operation is desired, the latch mechanism retracts the plunger. The lost motion arm is now allowed to pivot freely with respect to the outer body.
Accordingly, the cam lobe now makes contact directly with the outer body. In most instances, separate cam lobes make contact with the outer body than the cam lobe which makes contact with the lost motion arm so that a phase change may automatically be placed into the valve train system.
The body of the rocker arm assembly of Diggs et al. has contact slider pad engagement with the cam lobe during the short duration operation. Although slider pad contact is acceptable, from a wear standpoint it is preferable to have rolling contact engagement between the short duration cam lobe and body.
A major consideration for using the Diggs et al. finger follower rocker arms is that lateral spaces available for placement of a dual lift position rocker arm assembly are extremely limited (especially for in-line multi-valve engines with two or more intake or exhaust valves per cylinder). Therefore, it is highly desirable that the lateral width of the rocker arm assembly be reduced without compromising any strength requirements of the assembly.
Another consideration for rocker arm assemblies is that the upward movement of the lost motion arm with respect to the outer body be limited and set as accurately as possible. Accordingly, the rocker arm design which can be machined at low cost while meeting stringent dimensional tolerance levels is desired when setting the angle stop of the lost motion arm with respect to the remainder of the outer body.
To make manifest the above delineated and other desires, a revelation of the present invention is brought forth. In a preferred embodiment, the rocker arm assembly provides a longitudinal extending body. The body engages adjacent a first end with an engine valve stem to activate the same. Opposite the first end, the body engages with a pivot fulcrum.
A lost motion arm is provided that pivotally connects to the first end of the body. The lost motion arm is spring-biased by torsion springs into engagement with a stop surface. For high lift and low lift operations it is preferred to keep the inner arm from contacting the high lift cam lobe base circle.
The lost motion arm has a forked body. An elongated aperture extends through the forks of the lost motion arm. A sleeve is press-fitted within the aperture and rotatably connects a roller which is mounted on the sleeve by a needle bearing arrangement. A shaft eccentrically passes through the aperture. The shaft is press-fitted through aligned apertures passing through the main body. On opposite extreme ends, rollers are rotatably connected on the shaft.
The shaft performs two major functions. First, it allows rollers to be rotatably connected with the main body which accordingly enhance the wear characteristic of the rocker arm assembly. This eliminates the sliding contact of the main body with a cam lobe, as brought forth in the prior rocker arm of Diggs et al. Second, through contact with the inner diameter of the lost motion arm sleeve, the shaft acts as an angular stop surface for the lost motion arm.
A preferred embodiment multi-positional rocker arm assembly according to the present invention is advantageous in that it has rollers rotatably connected with the main body for rolling contact with the cam lobe when the rocker arm assembly is being utilized in the low lift mode of operation.
Additionally, the preferred embodiment of the present invention is also advantageous due to the angular limits of the motion of the lost motion arm with respect to the outer body. Accurately limiting the angular displacement of the lost motion arm downward with respect to the outer body helps eliminate the condition referred to as submarining. Submarining occurs when the lost motion arm is inadvertently held underneath an extended plunger, which is more likely when the lost motion arm swings higher than a maximum desired amount. If submarining occurs, the rocker arm assembly is locked permanently in low lift operation.
The shaft also serves as a stop to limit the engagement of the arm (via a slider pad or a roller) from contacting the cam lobe base circle. The dimensions and tolerance of the rocker arm assembly and the camshaft are controlled to ensure that the high lift cam lobe base circle never contacts the high lift follower (which is the lost motion arm slider pad or roller surface). As the lash adjuster compensates for tolerances and thermal effects, the lost motion arm remains at a nearly constant distance from the high lift cam base circle. This configuration is required to allow the roller on the lost motion arm to freewheel during low lift operation without having the lash adjuster pump up improperly.
Other features and advantages of various embodiments of the present invention will become more apparent to those skilled in the art from a reading of the following detailed description and upon reference to the drawings.